This was mentioned a while ago but some thoughts just germinated in my head:

To be a dragon rider you take a Dragon Rider edge which gives a small, recently hatched dragon, only big enough to carry the rider and their personal gear. Other than teeth and claws the dragon starts with no special weapons

Dragons advance using edges based on those for the Drakin in Sundered Skies allowing them to get bigger, gain breath weapons, learn skills etc, essentially customising the dragon as it grows and the rider advances.

For other setting rules Pirates pretty much covers things

Another method might be to use Clint's idea and build the dragon using the Necessary Evil Power Points system

For Dragon to Dragon warfare, consider In Harms Way: Dragons! I am not fond of the main system for characters, but I've heard the dragon fighting subsystem is very well done._________________Roll For News a podcast about what's new and noteworthy in Tabletop RPGs